PPS resin is excellent in properties including heat resistance, chemical resistance, flame resistance, and electrical resistance, and thus it is favorably used in engineer plastics, films, textiles, nonwoven fabrics, and so on. In particular, PPS long-fiber fabrics have been expected to be used in industrial applications such as heat-resistant filters, electrical insulation materials, and cell separators by fully utilizing these properties.
On the other hand, PPS long-fiber fabrics have disadvantages including poor dimensional stability against heat and extensive thermal shrinkage of fiber or nonwoven fabric. Until now, various proposals have been made to provide techniques for improving the dimensional stability of fiber or nonwoven fabric against heat.
For example, one of the proposed methods for obtaining a long fiber fabric comprises spinning PPS resin into a fiber by spin-bonding to make a fabric cloth; stretching it at a glass transition temperature or higher; and embossing the fabric preferably after biaxial stretching (see Japanese Unexamined Patent Application Publication (Kokai) No. 2005-154919). In addition, another proposed method for producing a long fiber fabric comprises spinning PPS resin by spin-bonding; subjecting the resulting fabric cloth to temporary bonding at a first crystallization temperature or less; and then subjecting the fabric to final bonding after heating at the first crystallization temperature or higher under strain (see Japanese Unexamined Patent Application Publication (Kokai) No. 2008-223209). However, as compared with a method for producing a spun-bonded nonwoven fabric from common resin such as polyester or polypropylene, the method for producing a nonwoven fabric from PPS resin by spin-bonding requires heat treatment equipment for thermal stretching or heat treatment of a nonwoven web or fabric cloth under strain. Thus, the method has a disadvantage in that the process is complicated and multi-staged, requiring large investment in equipment, while causing an increase in energy consumption.
Furthermore, there is another proposal for making the heat treatment equipment unnecessary by improving dimensional stability in the step of spinning PPS resin. For example, as a technique for improvement by resorting to raw materials, there is a proposed method for improving thermal dimensional stability by copolymerization of PPS resin with trichlorobenzene and then spinning and stretching the copolymerized product (see Japanese Patent No. 2890470). However, that proposal has a disadvantage in that the copolymerization with trichlorobenzene causes a decrease in spinnability and thread breakage frequently occurs at the time of spinning and stretching, thereby resulting in a lack of production stability.
As a technique for improvement in a spinning process, there is a proposed method for producing a heat-resistant nonwoven fabric with restrained thermal shrinkage by spinning PPS resin at a high spinning velocity of 7,000 to 11,000 m/min to improve the crystallinity of the fibers without stretching or heat treatment under strain (see WO 2008/035775). In that method, however, the high-speed spinning causes an increase in deformation of the fibers. Thus, the fibers tend to be broken frequently as the fibers are intolerable to deformation. In addition, an increase in energy consumption occurs because a large volume of compressed air is demanded. Furthermore, there is another proposed method for producing a PPS-spun-bonded nonwoven fabric by using air heated at a temperature of 80 to 100° C. to stretch PPS resin at a compressed air flow velocity of 300 to 3,000 m/min (see China Patent Application Publication No. 101532212). In that method, however, the spinning velocity is presumed to be less than 3000 m/min from the compressed air flow velocity. Thus, due to insufficient dimensional stability, there is the need of carrying out heat treatment under stain as a post-process in spite of excellent spinning stability.
Therefore, a method for producing a PPS long-fiber nonwoven fabric with excellent dimensional stability by simplified steps with stable spinnability has not been proposed.
It could therefore be helpful to provide a PPS long-fiber nonwoven fabric with excellent dimensional stability against heat by simplified steps with stable spinnability without subjecting a nonwoven fabric web to heat treatment in a post-process.